1. Field of the Invention
Alloys based on doped intermetallic compounds are acquiring increasing importance in materials technology. This is primarily due to the fact that numerous alloys based on a doped intermetallic compound, in particular an aluminide, are notable for high strength despite a low density. A difficulty in the case of such alloys is, however, a ductility which is inadequate for numerous applications.
2. Discussion of Background
In this connection, the invention proceeds from a prior art such as that which emerges, for instance, from the paper by Young-Won Kim entitled High-Temperature Ordered Intermetallic Alloys IV, "Recent Advances in Gamma Titanium Aluminide Alloys", Symposium Nov. 27-30, 1990, Boston, Mass. USA (MRS Proc. Vol. 213, pages 777-794).
From the prior art, it is known that those properties of an intermetallic compound which are critical for the application as a material for thermally stressed components are decisively determined by the microstructure and the grain size. In the case of an intermetallic compound based on doped gamma-titanium aluminide, the material structure determined by the microstructure and by grain size very substantially influences, in particular, the room-temperature elongation at break and the creep strength at the high temperatures to which components, such as, in particular, gas-turbine blades or turbine wheels of turbo chargers, made of such materials are exposed. Fine-grained duplex microstructures having mean grain sizes of approximately 20 .mu.m yield room-temperature elongations at break of typically up to 2%. Materials having such duplex microstructures have, however, a comparatively low creep behavior and are accordingly not particularly suitable as blade material for gas turbines. On the other hand, coarse-grained microstructures composed of lamellae having mean sizes of typically approximately 500 .mu.m have only a very low elongation at break of typically approximately 0.4% at room temperature, the creep behavior of a material having such a microstructure is nevertheless very good.
Hitherto, however, it has not yet been possible to produce materials which are based on doped intermetallic compounds having optimum microstructure and which have adequate ductility and also strength for use as gas-turbine blades.
In the production of a material based on, for example, gamma-titanium aluminide as intermetallic compound, a material having coarse-grained microstructure and a lamellar structure is formed if a casting method is applied. Although such a material is very creep-resistant at high temperatures, it has a very low ductility at room temperature.
Forging and shaping the cast material yields a dynamically recrystallized, fine-grained duplex microstructure having substantially improved ductility but also having substantially reduced creep properties. Such a duplex microstructure often has, in addition, inhomogeneities which take the form of bands.
The production of a material based on gamma-titanium aluminide by powder-metallurgy methods yields, after hot isostatic compacting and heat treatment, a material having either a fine-grained or a coarse-grained microstructure. Depending on the type of microstructure, such a material has either an unduly low creep strength or an unduly low ductility.